From the time the pussy willows open in early spring until the last of the asters, spring, summer, and fall are the seasons of flowers. They are glorious – but why do they exist at all? Like the singing of birds and the taste of ripe strawberries, they give us great pleasure, and for some that’s enough – but flowers are essential to the lives of flowering plants. Blooming is the culmination of months or years of growth, and understanding their parts will illuminate what they do.
Flowers have four kinds of parts, and they are in predictable positions. If we look into a flower as a pollinator might, we see small but important parts first. In the center is the female pistil, which forms the seeds and catches the pollen. The pistil has an ovary at its base, which contains one to many ovules, the tiny eggs that develop into seeds when fertilized. The pistil is topped by its stigma, which is sticky or feathery and situated where it will collect pollen. Between the stigma and the ovary is a stem, or style.
Also in the center, arranged around the pistil, are the male stamens, which produce pollen. A stamen has two parts: a slender stem, called the filament, topped by an anther, which is a small capsule that opens when the abundant pollen is ripe. A grain of pollen has a sturdy, often intricately sculpted coat and contains two cells – one that will form the pollen tube, and one that will produce the two sperm cells it takes to pollinate a single ovule. A few types of flowers, the milkweeds and the orchids, package pollen in a pollinium, a sticky container that attaches itself to a visitor and is transported whole to the next flower. Some flowers, including tomatoes, are buzz-pollinated – their anthers release pollen when they receive vibrations at just the right frequency, provided by a bumblebee.
The most important flower parts are the pistils and the stamens, and a flower must have one or both of those. But they aren’t the only parts. Like bees, butterflies, and birds, we humans are apt to notice the petals – they are the flower’s visual advertisement. Petals, collectively called the corolla, may be separate from each other or combined into a tube. Petals are of a scale, color, and shape to be visible to their pollinators. They may have surfaces that make it easy for pollinators to grip, contrasting centers like targets, and spots or stripes called bee guides that help insects find the nectar and the pollen, the nutrients they are after, more quickly.
Nectar is the sweet fluid produced by flowers. It has no function for the flower except to reward visitors. Many insects, hummingbirds, and pollinating bats relish nectar as a high-energy food; honeybees and bumblebees use it to make honey. Along with color, many flowers produce scents, which waft through the air and can attract specific pollinators. Many flowers smell sweet to us, like wild roses and milkweed, but we find others strange and pungent or disgusting. Red trilliums are dark red like dried blood and smell like rotting meat. They bloom early in the spring and are pollinated by carrion-eating flies, which are out looking for newly thawed carcasses to lay their eggs in. They don’t find a spot for their eggs, but they do provide pollination services to a patch of trilliums.
Sepals are the outermost ring of flower parts. In most flowers, they are green like leaves and serve to protect the flower as a bud and when it closes in the rain or dark. They can be separate or tubular, and they may fall off as soon as the flower opens, as in poppies and bloodroot, or persist at the base of the fruit as it develops. Together, they are called the calyx. It’s not uncommon, though, for sepals to be brightly colored and petal-like. When sepals and petals are so alike that they’re hard to tell apart, as in a tulip, they are called tepals.
Seeds are formed when pollen and ovule cells of the same species combine in the process of pollination. We can see pollen, see that it lands and sticks on the flower’s stigma, but how does it get to the ovule, deep inside the ovary? It’s a strange process. The pollen sprouts, breaking out of its tough shell, and sends a shoot down the style. The shoot carries male cells that eventually get to the ovule, combine with it, and start the maturing of the seed.
After pollination, seeds form, and the flower is transformed into what botanists call a fruit. In everyday speech, a fruit is a tasty, soft enclosure for seeds, such as an avocado, an apple, a blueberry, or a grapefruit, but in plant biology, walnuts, maple keys, and milkweed pods are all fruits. They consist of the plant’s ripened ovary and the seeds inside it. When we take the pistil apart, we can see the arrangement of the ovules within the ovary. The ovary may be superior, above the other flower parts, as in a pepper, eggplant, or pomegranate, or it may be inferior, below the flowers, like a pumpkin or apple. If you look at a fruit, you can distinguish the stem and blossom ends and often see vestiges of the flower parts.
We classify plants by the number and arrangement of flower parts because the flowers give us the best clues to their relationships. There are about 350,000 species of flowering plants in over 400 families in the world. The grass, orchid, and aster (including daisies and dandelions) families are the largest; other familiar flowers, herbs, fruits, and vegetables belong to the rose, buttercup, violet, bean, squash, carrot, and mint families, to name a few. Learning to recognize common families is a rewarding study and a good way to get to know the plants.
Among the first plants to bloom in the spring are trees with light, dry pollen in abundance, like the poplars and birches. These are wind-pollinated; the wind carries their pollen everywhere, including, by chance, to the female flowers of those plants. Since the leaves are not yet out on the trees, the pollen has a much better chance of reaching its goal. As well as many trees, the grasses, sedges, and some plants in other families are also wind-pollinated. Their flowers are inconspicuous, built to shed and catch pollen in the air rather than to attract pollinators.
Wind might be the simplest pollinator, but it’s not the most common, and it can’t carry the big, sticky pollen that many flowers produce. There’s a wide cast of characters, from pollen and nectar-eating insects of many kinds to tropical bats and hummingbirds, that performs this task, though eighty-five percent of it is done by bees – bumble bees, solitary bees, and domestic honeybees. Bees feed pollen to their larvae, so they are motivated to visit one flower after another. This makes them very efficient pollinators compared to a beetle, fly, butterfly, or moth, insects that just want a meal for themselves.
Many plants have ways to make sure that the pollen reaching the ovules is from another flower of the same species and not its own pollen. To ensure cross-pollination, some flowers have the stigma and the pollen ripen at different times; other species have their flowers on separate plants or in some way inaccessible to each other. In some plants, the cells of the stigma won’t allow its own pollen to germinate. It’s one of the complexities of the plant kingdom that some flowers do self-pollinate. Dandelions, violets, beans and peas, and many others can be cross-pollinated but use self-pollination most of the time. While this doesn’t have the advantages of outbreeding, which could help offspring survive in changing conditions, it does ensure a full crop of seeds for the next generation.
From spring to fall we feast our eyes on a profusion of flowers while pollinators dine upon their bounty. Flowers adorn our gardens; we paint their portraits, write and sing about them. And what a fascinating world we find when we look at them closely. The intricate, varied, and complex structures of flowers have all evolved to ensure the production of seeds, no matter what. Because that’s what flowers are for.
Burns, Deborah, ed. Attracting Native Pollinators: Protecting America’s Bees and Butterflies. The Xerces Society, 2011.
Hickey, Michael, and Clive King, Common Families of Flowering Plants, Cambridge Press, 1997.
Newcomb, Lawrence, Newcomb’s Wildflower Guide, Little, Brown and Company, 1989.
Proctor, Michael, Peter Yeo, and Andrew Lack, The Natural History of Pollination, Timber Press, 1996.